The present invention relates to an optical time-domain reflectometry apparatus capable of measuring characteristics such as optical losses at any point along the longitudinal direction of an optical fiber by launching a light pulse into the optical fiber from its one end and measuring the intensity of the backward scattering light with the passage of time.
In an optical time-domain reflectometry (which will be abbreviated as OTDR hereinafter) apparatus, the loss distribution along the longitudinal direction of an optical fiber can be acquired by the following procedures. A light pulse generated from a light source is launched into an optical fiber under test from one end thereof by means of an optical coupler or optical switch. The light scattered inside the optical fiber under test, returning backward and outgoing from the aforementioned one end (known as backward scattering light), is split from the incident light by means of the optical coupler or optical switch. The backward scattering light, thereafter, is led to an optical receiver to be transformed into electric signals, and thus the intensity of the backward scattering light is obtained in correspondence with the passage of time. The variations in the intensity of the backward scattering light intensity are analyzed with respect to the passage of time from the time at which the light pulse is generated, in order to obtain the loss distribution along the longitudinal direction of the optical fiber.
Since the intensity of the Rayleigh scattering light detected as the backward scattering light is weaker than one-ten thousandth that of the incident light, use of a laser diode (will be abbreviated as LD hereinafter) which is compact and has a high intensity of light has been made. For this light source, the LD devices respectively having wavelengths of 0.85 .mu.m, 1.3 .mu.m, and 1.55 .mu.m are usually employed since these have been brought into commercial use for communication equipment.
The conventional OTDR apparatus, however, has a problem that it is impossible to measure the loss with respect to arbitrary wavelengths in the wide wavelength range, except at 0.85 .mu.m, 1.3 .mu.m, and 1.55 .mu.m. In other words, an LD with a wavelength other than the aforementioned wavelengths is hard to use practically since it is not available due to the lack of production, or due to extremely high cost.
For these reasons, it is impossible to measure the loss increase at a wavelength of 1.38 .mu.m which may arise when the optical fiber includes impurities such as water or the like. Accordingly, it is impossible to determine whether or not an optical fiber has any locally increased loss by being mixed with impurities in the course of manufacturing, and whether or not an optical fiber has any locally increased loss by water breaking thereinto when the optical fiber installed is submerged. Consequently the conventional apparatuses have difficulty evaluating important factors of optical fibers.
On the other hand, there has been provided another conventional method which makes it possible to measure the loss characteristic of an optical fiber in a wavelength range other than 0.85 .mu.m, 1.3 .mu.m, and 1.55 .mu.m; in such a manner that by leading light having a certain wavelength to one end of the optical fiber with the help of a white light source and spectroscope, the output light from the other end is detected to measure the total loss along the longitudinal direction of the long optical fiber. However with this method, the measurement cannot be made on the loss at each point along the longitudinal direction of the optical fiber, so that it is impossible to measure the loss caused by impurities existing locally along the longitudinal direction.